Biological Nitrogen Fixation in CMIP6 Models

Abstract. Biological nitrogen fixation is the main source of new nitrogen into natural terrestrial ecosystems and consequently in the nitrogen cycle in many earth system models. Representation of biological nitrogen fixation varies, and because of the tight coupling between the carbon and nitrogen cycles, previous studies have shown this affects net primary productivity. Here we present the first assessment of the performance of biological nitrogen fixation in models contributing to CMIP6 compared to observed and observation-constrained estimates of biological nitrogen fixation. We find that 9/10 models represent global total biological nitrogen fixation within the uncertainty of recent global estimates. However, 6/10 models overestimate the amount of fixation in the tropics, and therefore the extent of the latitudinal gradient in the global distribution. For the SSP3-7.0 scenario of future climate change, models project increases in fixation over the 21st century of up to 80 %. However, while the historical range of biological nitrogen fixation amongst models is large (up to 140 kg ha−1 yr−1 at the grid cell level and 43–208 TgN yr−1 globally) this does not have explanatory power for variations in net primary productivity or the coupled nitrogen-carbon cycle. Models with shared structures can have significant variations in both biological nitrogen fixation and other parts of the nitrogen cycle without differing in their net primary productivity. This points to systematic challenges in carbon-nitrogen model structures.